The xerographic imaging process begins by charging a photoconductive member to a uniform potential, and then exposing a light image of an original document onto the surface of the photoconductor, either directly or via a digital image driven laser. Exposing the charged photoconductor to light selectively discharges areas of the surface while allowing other areas to remain unchanged, thereby creating an electrostatic latent image of the document on the surface of the photoconductive member. A developer material is then brought into contact with the surface of the photoconductor to transform the latent image into a visible reproduction. The developer typically includes solid particles with an electrical polarity opposite to that of the photoconductive member. A blank copy sheet is brought into contact with the photoreceptor and the toner particles are transferred thereto by electrostatic charging the sheet. The sheet is subsequently heated, thereby permanently affixing the reproduced image to the sheet. This results in a "hard copy" reproduction of the document or image. The photoconductive member is then cleaned to remove any charge and/or residual developing material from its surface to prepare it for subsequent imaging cycles.
The solid used to develop the electrostatic latent image is commonly referred to as toner. Toner is typically a dry, powdery substance, which, as indicated above, has an electrical polarity opposite to that of the latent image. Toner is often manufactured using an extruder with an extruder die present at the end where the toner exits. The main purpose of this die is to shape the molten extrudate into the proper form, which depends upon the cooling mechanism that is in place. The base resin from which the toner is made may have a significant impact on the overall quality of a finished copy. The resin may also have an impact on the life of certain mechanical components of the copier or printing machine in which it is used. Further, the cost of producing toner in a manufacturing facility is related to the type of base resin from which the toner is made. Polyester resins, typically manufactured using a process known in the art as "reactive extrusion" has been proven to have a positive impact on each of the relationships stated above. As an added benefit, polyester resin that has been produced using reactive extrusion can be crosslinked (polymer molecular chains intertwined to form a network) during the extrusion process. Crosslinking provides anchoring points for the polymer molecular chains thereby restraining excessive movement and maintaining the position of the chains within the network. This results in improved material qualities such as added dimensional stability, lowered creep rate, resistance to solvents, and less heat distortion. However, crosslinking of a polyester resin can also produce a viscoelastic polymer with considerable elastic properties. Highly elastic viscoelastic polymers are known to have a relatively high Deborah number, the ratio of the relaxation time of a fluid to an appropriate time scale of deformation. The characteristic of relatively high Deborah number is evident in viscoelastic polymers with high elasticity or high elastic modulus.
In the manufacturing process described above, the molten extrudate from which the xerographic toner is made may expand or "swell" as it exits the die, such that the cross sectional area of the final product will exceed that of the die exit. This expansion of the extrudate as it leaves the die, referred to as "die swell," is a phenomenon, which is experienced to a higher degree in polymers that have considerable elastic properties or relatively high Deborah number. When using a cooling mechanism known as a belt cooler, die swell prevents the extrudate from easily passing through the nip between transport rolls, which move the extrudate onto the belt cooler. Thus, it is advantageous to eliminate or minimize die swell in order to efficiently manufacture high quality xerographic toner when the resin used is highly elastic.
The following disclosures may be relevant to various aspects of the present invention:
U.S. Pat. No. 5,234,652 to Woodhams et al. issued Aug. 10, 1993 discloses a process for the continuous production of high modulus articles from high molecular weight plastics which includes forcing a high molecular weight plastic material through a passage of which the cross-sectional area diminishes in the forward direction of plastic flow, thus producing an extrudate. The plastic material is extruded while it is close to or at its melt temperature, and it is lubricated to obtain substantially plug flow through the passage. The speed at which the plastic material flows through the passage is adjusted to control the elongational velocity gradient at any longitudinal position within the passage, thus producing an extrudate that contains with substantially no fibrillar molecular orientation. The extrudate can be deformed by drawing while it is maintained at or close to its melt temperature, thus producing an oriented, deformed extrudate. The oriented extrudate is then quickly cooled to preserve the orientation.
U.S. Pat. No. 4,859,379 to Chiang issued Aug. 22, 1989 discloses a process for forming film from a melted polymer by extruding the polymer through a slot-die under high a Deborah number condition. The extruded film is then heated while it is in the molten state, to produce a film, which is more uniform in thickness than a film prepared under the same conditions but without heating. The film is cooled, and drawn in the machine direction, thereby reducing draw resonance.
U.S. Pat. No. 4,865,796 to Tamura et al. issued Sep. 12, 1989 discloses a method of producing molding members, which are formed at least partly of a synthetic resin material. A synthetic resin material is extrusion molded into a continuous body with a substantially constant cross-section throughout the entire length thereof. A controlled amount of the material is removed from the predetermined location of the continuous body, synchronously with the extrusion molding of the material, such that the cross-section of the continuous body varies in the longitudinal direction of the body. The continuous body subjected to the controlled removal of the material is then cut into the predetermined length of the molding member.
Middleman, Stanley: "Fundamentals of Polymer Processing" McGraw-Hill, 1977, pg. 61-62, 468 contains a short explanation of the die swell phenomenon as it relates to the behavior of a viscoelastic polymer.
All of the references cited herein are incorporated by reference for their teachings.
Accordingly, although known apparatus and processes are suitable for their intended purposes, a need remains for manufacturing xerographic toner. More specifically, there is a need for processes and apparatus, which allow highly elastic viscoelastic polymers which exhibit relatively high Deborah numbers to be extruded without suffering from die swell. Further, there is a need for processes and apparatus' for manufacturing xerographic toner from materials that have low melting temperatures, which can result in lower power consumption for the printer or copier being used, and an increase in the life of certain fuser components. In addition, a need remains for process and apparatus for producing xerographic toner that reduces or eliminates the phenomenon known as "vinyl off set," which causes the transfer of a hard copy image onto a vinyl material.